The present invention relates to low alternating current (AC) loss high temperature superconducting coils, to methods of fabricating such superconducting coils and to devices which utilize high temperature superconductor [HTS] tape coils such as transformers, motors, generators, etc.
Electrical conductors, such as copper wires, form the basic building block of the world""s electric power system, i.e., wire in transformers, electric motors, generators, and alternators. The discovery of high-temperature superconducting compounds in 1986 has led to the development of their use in the power industry. This is the most fundamental advancement in conductor technology used for power systems in more than a century.
Over the past three decades, electric power use has risen about 25%-40% in the United States. With this rising demand for power comes an increased requirement for low-cost power. Because of the lack of DC resistance and the low AC losses of superconductors at operating temperatures, superconducting devices are being developed for application throughout the electric power industry.
The power industry""s future use of superconductors depends on the overall cost and performance (low power loss) benefits that the superconductor wires offer. HTS tape technologies drive down the costs, increase the current-carrying capacity, and improve the reliability of the wiring system, thus impacting electric power systems in a variety of ways. These ways include the possibility of greatly reduced size and weight of the wires used in devices such as transformers, motors, and generators. Superconductor wires have many applications because of their efficiency for carrying electricity and their ability to carry much higher electrical currents than other conducting materials in less volume.
There exists the unmet technical challenge in the power industry of fabricating HTS coils and devices in such a way that they operate with negligible alternating current (AC) losses. These superconductors can carry direct current (DC) with negligible losses, but DC is rarely used in the power industry. AC is the dominant form in most of the world""s power coil-based devices. AC applications of HTS tapes operate with non-negligible energy losses, the energy escaping in the form of heat. This impacts the efficiency of the system beyond the mere energy loss since the heat generated must be removed from the environment of the device.
Superconductors operate in the temperature range of 4xc2x0-85xc2x0 K, far below ambient temperature (298xc2x0 K). Thus, superconductors require refrigeration, and refrigeration requires continuous expenditure of energy. For example, if the heat caused by the electrical current flowing in superconductor wires is at 77xc2x0 K and is dissipated at the rate of one watt, then refrigerators must be supplied with approximately 10-40 watts of electrical power to dissipate that generated heat. Absent this refrigeration, the superconductor material would warm itself to above its sukerconducting temperature and cease to operate as a superconductor, thereby eliminating any advantage and, in particular, providing worse performance than conventional copper conductors.
The heat generated must be eliminated to cost-effectively maintain the low temperatures required by the superconductor. Successful solution of this problem would reduce operating costs by reducing the added cooling energy needed.
The key problem of HTS tapes is that unwanted AC magnetic fields are generated by the current flowing in the neighboring HTS tapes, which causes AC losses. Because the HTS tape material and geometry is anisotropic, magnetic fields passing perpendicular to the preferred direction generate significantly greater losses than those of parallel fields. In the present invention, there are no perpendicular magnetic fields except for the very ends of the wiring structures, where different loss mechanisms apply. A discussion of AC losses caused by magnetic fields can be found in W. T. Norris, J. Phys. D 3 (1970) 489-507, or Superconducting Magnets by Martin N. Wilson, Oxford University Press, Oxford, UK 1983.
Kalsi et al., U.S. Pat. No. 6,081,987, entitled xe2x80x9cMethod of Making Fault Current Limiting Superconducting Coil,xe2x80x9d provides a multiple tape HTS system. Kalsi et al. describes a superconducting magnetic coil that includes a first superconductor formed of a first anisotropic superconducting material wire for providing a low-loss magnetic field characteristic for magnetic fields parallel to the longitudinal axis of the coil, and a second superconductor material wire having a low-loss magnetic field characteristic for magnetic fields, perpendicular to the longitudinal axis of the coil. The first superconductor has a normal state resistivity characteristic conducive for providing current limiting in the event that the second superconductivity wiring material of the magnetic coil is subjected to a current fault.
Kalsi et al. wires two superconductive HTS wiring tapes in parallel along the length (longitudinally) of the cable, but the two HTS wiring tapes are of different materials and one HTS wiring tape is used as a back up for fault tolerance. There is no mention of wiring configurations to reduce AC losses.
It would be highly beneficial to develop a superconductor configuration that reduces AC losses and associated very high refrigeration costs. Practical devices for AC applications could then be wound using wide flat superconductors, the most prevalent and desirable form of high temperature superconductors (HTS).
Thus, it is an object of this invention to provide a method of fabricating superconductor coils such that AC losses due to the presence of a localized perpendicular component of the self-field is eliminated or minimized.
It is another object of this invention to provide superconducting coils with minimized AC losses due to the presence of a localized self-field perpendicular field component.
It is yet another object of this invention to provide superconducting devices with minimized self-generated AC losses.
It is yet another object to reduce refrigeration requirements associated with the operation of a HTS tapes used in wiring coil-based devices by reducing the heat generated by perpendicular magnetic fields impinging on neighboring HTS tapes.
It is yet another object of this invention to use conventional HTS wiring tapes and conventional wiring methods in a new wiring configuration to create a low cost superconducting device.
HTS tapes may be wound around coil structures in various ways described as xe2x80x9cwinding configurationsxe2x80x9d. Winding configurations can be changed in a variety of ways by changing (1) the size of the superconductor wires (width, thickness, shape) on the coil structure, (2) the type of superconductor material used, and (3) the way the tape is wound on a coil structure itself (spacing to its neighboring wire).
Surprisingly, it has been determined that eliminating the gaps normally present when superconductor tapes are wound into coils prevents significant energy losses and limits the need for cooling of the superconductor. The present invention obtains low AC loss results by providing novel techniques of winding the tape on a coil stricture.
In most applications, the HTS tape is continuously in the presence of an AC field. The present invention is directed toward HTS tape-winding configurations used in applications where the AC frequency is typically in the range of 50-60 Hz (normal operating frequency in the power industry). By using HTS tapes instead of standard copper wires, better performance (lower power losses) and lower cost are achieved. However, HTS tapes require cooling, which uses power. The present invention is directed to HTS tape wiring configurations designed to achieve low AC losses, thereby reducing refrigeration requirements and enabling superconducting wiring structures to achieve their higher performance at lower cost.
A significant source of AC loss is the loss caused by the magnetic fields of the neighboring HTS tapes, said field being generated by AC current traveling through HTS tapes. In particular, the magnetic self-fields that are allowed to form because of gaps between the HTS tapes.
It has now been discovered the superconductors composed of conventional materials but wound in specified configurations eliminate certain energy losses commonly present in HTS applications. The invention applies broadly to a superconductor winding configuration that eliminates local perpendicular field components.
This new HTS tape configuration approximates a single current xe2x80x9csheetxe2x80x9d, which produces minimal magnetic fields perpendicular to the current flow, thus significantly reducing AC losses.
The invention comprises a method of fabricating superconductor coils that minimize the AC losses in the main body of the superconducting coil and low AC loss superconducting coils. The beneficial results of the invention are obtained by fabricating superconducting coils such that superconductors overlap one another so that gaps between the superconductors are covered by another superconductor.
Because there are no uncovered gaps, the individual turns of the HTS tapes approximate a single long former of current, forcing the magnetic field to be primarily parallel to the surface of the former and surface of the superconductor. This is a preferential orientation because it minimizes or eliminates the component of the magnetic field perpendicular to the surface of the superconductor. With no substantial perpendicular field component, the high perpendicular field losses in the superconductor are eliminated.